Digital cameras use a lens to focus a scene onto an image sensor. Many digital cameras include an auto-focus system which can be used to automatically adjust a focus setting for the lens to achieve sharp focus for a main subject in the scene. Other digital cameras provide a manual focus capability which enable a user to manually adjust the focus setting.
All variable focus digital camera lenses will have an inherent focus distance range defined by a “near focus position” corresponding to the smallest subject distance at which the lens can be focused and a “far focus position” corresponding to the largest subject distance at which the lens can be focused. Generally, the far focus position will correspond to an infinite subject distance.
Real lenses do not focus all light rays perfectly, so that even at best focus, a point in the scene is imaged as a “blur circle” rather than a point. At any given focus distance within the focus distance range, there exists a “depth of field” corresponding to a range of subject distances around the focus distance where objects in the scene will be acceptably focused. The depth of focus will correspond to the subject distances where the size of the blur circle for the lens is less than a maximum permissible size. The maximum permissible size will correspond to the point where increasing the blur circle size begins to degrade the visual quality of the resulting image.
As is well-known in the art, the depth of field for a lens will depend on the F/# of the lens. A smaller F/# (corresponding to a larger aperture diameter) will have a smaller depth of field, while a larger F/# (corresponding to a smaller aperture diameter) will have a larger depth of field.
To reduce costs, some digital cameras use a fixed focus lens, which does not have an adjustable focus setting. Fixed focus lenses can also provide other advantages such as increased system robustness, reduced shutter lag, and reduced lens size. Fixed focus lens are generally focused at a “hyperfocal distance” for the smallest available F/#. The hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp. Therefore, fixed focus lens generally have a depth of field that extends from infinity to some close focus distance. When the lens is at the smallest F/#, the close focus distance is typically half of the hyperfocal distance.
Sometimes a photographer may desire to capture an image of an object at a subject distance that is closer than the near focus position of the lens. This is particularly problematic for digital cameras having fixed focus lenses. While a skilled photographer may be able to manually adjust the F/# of the lens such that the desired object falls within the depth of field, such manipulations are beyond the ability and understanding of most amateur photographers.
U.S. Patent Application Publication 2010/0128137 to Guidash, entitled “Extended depth of field for image sensor,” discloses a method for achieving an extended depth of field for an imaging device including an imaging sensor. The method involves capturing two different images using different F/# settings, and then forming a composite image by combining image data from the two images.